I am starting from a place of having no gadgets on my bike at all...no computer, nothing...

If I would like to have the relevant info (Speed, cadence, power, etc)...what's the cheapest way to get there?

Computer first, then add power-meter?

Or is there a Power Meter that comes with a computer read-out that will also tell me my speed, that would be more economical?

How much should I expect to spend, if I want to spend the least possible?